Kenco makes a great seam sealer... they make a water based kind (Seam Sealer 3) for just sealing seams, and a urethane kind (SeamGrip) that's good for seam sealing and for minor repairs. I've had good luck with the water based (as odd as that may sound). It goes on fast and dries fairly quick. The Seam Sealer 3 can be found at Wal-Mart too... look for the tent stakes -- it'll be close by. You'd probably have to go to a real outdoor sporting goods store (REI, Dick's, etc.) to find Seam Grip.
As far as waterproof durability, I generally give mine a good coat or two of the waterproofer and re-do the seam sealer at the start of every spring. It's always lasted at least until the next spring. I haven't had any water in the tent that I didn't bring in with me (knock on wood!)
And yes, I spend WAAAY too much time at Wal-Mart... and too much money too, but who can resist?